Inspired by a thread on HackerNews, I've compiled a list of websites where you can post your project to get some initial feedback from beta users (also used the answers in this quora answer):
http://www.reddit.com/r/sideproject
http://www.romanianstartups.com
https://www.sideprojectors.com
http://momb.socio-kybernetics.net
http://projects.metafilter.com
http://www.geekopedia.me/startupsubmit
http://www.thestartuppitch.com
http://www.feedmyapp.com/submit
http://www.siliconallee.com/contact
http://www.aboutyourstartup.com
http://www.eu-startups.com/directory
http://www.101bestwebsites.com
http://www.venturebeatprofiles.com
http://www.launch.it/contact_form/1/0/contact
http://www.netted.net/contact-us
http://www.minisprout.com/about
http://www.makeuseof.com/contact-team
http://www.venturevillage.eu/about-us/contact
http://www.webdevtwopointzero.com
http://www.techattitude.com/contact
http://www.en.startupbusiness.it
http://www.blogs.wsj.com/tech-europe
http://www.en.webrazzi.com/about
http://www.sociableblog.com/contact-us
http://www.erlibird.com/startups
https://starthq.com/apps/submit
http://sideprojects.web3canvas.com
start-ups.co/submit-your-start-up
Update 2013-10-28: